1. Field of the Invention
The present invention relates to a positive resist composition for use in lithographic steps in the production of semiconductors, e.g., IC's, in the production of circuit boards for liquid crystals, thermal heads, etc., and in other photofabrication processes, and further relates to a method of pattern formation with the same. In particular, the invention relates to a positive resist composition suitable for exposure with an immersion type projection exposure apparatus employing far ultraviolet rays having a wavelength of 300 nm or shorter as an exposure light, and to a method of pattern formation with the composition.
2. Description of the Related Art
With the trend toward size reduction in semiconductor elements, the wavelengths of exposure lights are decreasing and the numerical apertures (NA) of projection lenses are increasing. An exposure apparatus which has an NA of 0.84 and employs an ArF excimer laser having a wavelength of 193 nm as a light source has been developed so far. As is generally well known, resolution and focal depth can be expressed by the following equations:(Resolution)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2 
wherein λ is the wavelength of the exposure light, NA is the numerical aperture of the projection lens; and k1 and k2 are coefficients relating to the process.
An exposure apparatus employing an F2 excimer laser having a wavelength of 157 nm as a light source is being investigated for the purpose of enhancing resolution by using a shorter wavelength. However, use of this apparatus is disadvantageous in that materials for the lens to be used in the exposure apparatus and materials for resists are considerably limited due to the use of such a shorter wavelength. Because of this, the cost of apparatus and material production is high and it is exceedingly difficult to stabilize quality. There is hence a possibility that an exposure apparatus and a resist which have sufficient performances and stability might be not available in a desired period.
The so-called immersion method has been known as a technique for enhancing resolution in examinations with optical microscopes. In this method, the space between the projection lens and the sample is filled with a liquid having a high refractive index (hereinafter referred to also as “immersion liquid”).
This “immersion” has the following effects. In the immersion, the resolution and the focal depth can be expressed by the following equations on the assumption that NA0=sin θ:(Resolution)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 wherein λ0 is the wavelength of the exposure light in air; n is the refractive index of the immersion liquid relative to that of air; and θ is the convergence half angle of the light.
Namely, the immersion produces the same effect as the use of an exposure light having a wavelength reduced to 1/n. In other words, in the case of an optical projection system having the same NA, the focal depth can be increased to n times by the immersion. This is effective in all pattern shapes and can be used in combination with a super resolution technique such as the phase shift method or deformation illumination method which is being investigated at present.
Examples of apparatus in which this effect is applied to the transfer of fine image patterns for semiconductor elements are shown in JP-A-57-153433, JP-A-7-220990, etc.
Recent progress in the immersion exposure technique is reported in SPIE Proc, 4688, 11(2002), J. Vac. Sci. Technol., B 17 (1999), SPIE Proc., 3999, 2 (2000), International Publication WO 2004-077158, pamphlet, etc. In the case where an ArF excimer laser is used as a light source, pure water (refractive index at 193 nm, 1.44) is thought to be most promising from the standpoints of safety in handling and transmittance and refractive index at 193 nm. Although solutions containing fluorine are being investigated for use in the case of using an F2 excimer laser as a light source from the standpoint of a balance between transmittance and refractive index at 157 nm, no immersion liquid has been found which is sufficient from the standpoints of environmental safety and refractive index. In view of the degree of the effect of the immersion and the degree of completion of resists, the technique of immersion exposure is thought to be employed first in ArF exposure apparatus.
Since the advent of resists for KrF excimer lasers (248 nm), the technique of image formation called chemical amplification has been used as a resist image formation method for compensating for a sensitivity decrease caused by light absorption. For example, the chemical amplification type method for forming a positive image comprises exposing a resist film to light to thereby cause an acid generator in the exposed areas to decompose and generate an acid, subjecting the resist film to post-exposure bake (PEB) to utilize the resultant acid as a reaction catalyst to convert alkali-insoluble groups into alkali-soluble groups, and removing the exposed areas by alkali development.
Resists for an ArF excimer laser (wavelength, 193 nm) which work by the chemical amplification mechanism are coming to be mainly used presently. However, use of these resists has a problem that a line pattern formed falls to give defects in device production. An improvement in this respect has been desired.
It has been pointed out that application of a chemical amplification type resist to immersion exposure arouses troubles that since the resist layer is in contact with an immersion liquid during exposure, the resist layer alters and that components which exert an adverse influence on the immersion liquid are released from the resist layer. International Publication WO 2004-068242, pamphlet described an example in which a resist for ArF exposure changes in resist performance upon immersion in water before and after exposure. It is pointed out therein that this change is a problem in immersion exposure.
In the case where exposure in an immersion exposure process is conducted with a scanning type immersion exposure machine, the immersion liquid should follow the movement of the lens. However, in case where the immersion liquid does not follow the lens, there is a fear that the speed of exposure may decrease to influence productivity. When the immersion liquid is water, the resist film desirably is hydrophobic because water on a hydrophobic resist film is more satisfactory in following-up properties. However, impartation of hydrophobicity to a resist film, on the other hand, results in adverse influences on the image-forming performance of the resist, such as an increased scum amount. An improvement in this respect has been desired.